Neurostimulation is a category of medical devices that are used to transfer electric charge or electrical fields to tissue and result in a physiological change which benefits the patient, or performs a physiological measurement. Neurostimulation is used today in the cochlea, the retina, the peripheral nerve system, the spine, the brain and other parts of the body.
In a particular application of Neurostimulation, conductive electrodes are placed in contact with certain deep brain structures in order to treat certain neurological conditions. In the case of stimulating the Subthalamic Nucleus, for example, as described in U.S. Pat. No. 5,716,377, or the Globus Pallidus, for example, as described in U.S. Pat. No. 5,683,422, the therapy can treat the symptoms of Movement Disorders such as Parkinson's disease, Essential Tremor or Dystonia. In the case of stimulating the cerebellum, Hippocampus and other brain structures, the therapy can treat the symptoms of Epilepsy [Theodore, W. H., Fisher, R. S., “Brain stimulation for epilepsy”, Lancet Neurology, 3 (2), pp. 111-118, (2004)].
An implantable pulse generator supplies the electrical signal to the electrode lead in contact with the brain structure. All components are placed surgically.
In most prior art the electrode placed in contact with the brain tissue has been metallic, cylindrical, and relatively large in size (e.g., 1.5 mm in length). In many cases, the electrodes are as large as the brain structures themselves. The large size of electrodes prevents specific and precise stimulation of small brain targets such as the pedunculopontine nucleus. The resulting large electric fields and associated current paths stimulate other structures of the brain, and do not concentrate on the intended target. Furthermore, these large electrodes cannot be used to identify the targets of the brain by neural-recording because the area they cover is very large.
Current techniques that determine placement of such relatively large electrodes are accomplished cutaneously by first inserting a relatively small (e.g., 600 μm diameter probe). The relatively small probe can be inserted along an approach near the target. Recordings of neural activity can be made as the probe is advanced along the approach until the intended target is located. The depth of the probe from a reference is recorded and the relatively large electrodes are inserted along the same trajectory, being placed at the recorded depth. This process is complex, requiring a highly skilled surgeon to place both the probe and later the electrode. Repositioning and removal of the probe and reinsertion of the electrode subject the patient to heightened risk as the risk of tissue damage and bleeding is increased.
Attempts have been made at developing microfabricated devices specifically designed to incorporate an array of microelectrodes which can stimulate small volumes of tissue in the deep brain, for example, as described in U.S. Pat. App. Pub. 2007/0118197, or “Multisite Microelectrodes for Use in Human Deep Brain Stimulation” by Hofmann et al., Microtechnologies in Medicine and Biology, 2006 International Conference on (2006) Pgs. 284-287. The prior devices however do not have a clear path to clinical use because they are too unfamiliar to the neurosurgeon performing the implantation procedure.
An important requirement for a successful outcome of deep brain stimulation (DBS) treatment, is the accurate placement of the stimulation electrodes within the stimulation target area. Mislocation may result in unwanted side-effects, including sensory motor effects and mood changes. Prior art procedures approximately localize the target by pre-surgical imaging and planning to identify a trajectory to minimize risk of damage. It may be impossible to locate the exact functional anatomy within a target region of the brain. The targets themselves may be only a few mm or less, and not detectable through standard imaging techniques alone. Also, position changes of the brain may occur when surgically opening the skull to implant the electrodes and when inserting the electrodes. Current procedures insert test electrodes used to perform electrophysiological exploration of the target area. Once the precise target area is located, the chronic stimulation electrodes can be implanted at the precise location.
Disadvantages to the current technology include extension of operation time by several hours, which can be an increased burden for the patient, who is typically awake during such procedures, and extended cost associated with lengthier procedures. Increased risk of surgical complications from bleeding or tissue damage caused by repeated insertion and extraction of test and chronic leads. Possibility that chronic leads are not precisely located at identified target for any number of reasons, including further brain movement. An increased chance of infection due to an open craniotomy for several hours.